Bandwidth reduction for video signals of low luminance

ABSTRACT

A circuit for receiving video signals from a television camera tube is described. The circuit includes a delay line which is reflective at one end and terminates in its characteristic impedance at its other end so as to be nonreflective. Video signals, derived from the television camera tube, are applied to the one end of the delay line through a diode biassed signallevel-dependent circuit adapted to differentiate against high level signals, and video signals are also applied to the other end of the delay line through a similar diode biassed circuit adapted to differentiate against low level signals. Means are provided for taking off output video signals from the end of the line which is terminated in the characteristic impedance of the line. The bandwidth for video signals corresponding to low luminance areas of a subject of transmission is reduced compared to the bandwidth for video signals corresponding to higher luminance areas.

United States Patent BANDWIDTl-l REDUCTION FOR VIDEO SIGNALS 0F LOWLUMINANCE 9 Claims, 3 Drawing Figs.

US. Cl l78/7.1, 178/6, 179/l5.55, 325/475 Int. Cl H04n 7/12, H04b 1/66Field of Search 325/427, 474, 475; 178/6 (VBW); 333/70 (T); 328/165;179/1555 [56] References Cited UNITED STATES PATENTS 3,352,969 11/1967Konings 178/6VBW 2,816,267 12/1957 Jager et a1 325/38.1

Primary ExaminerR0bert L. Griffin Assistant Examiner-Donald E. StoutAttorney-Baldwin, Wight & Brown ABSTRACT: A circuit for receiving videosignals from a television camera tube is described. The circuit includesa delay line which is reflective at one end and terminates in itscharacteristic impedance at its other end so as to be nonreflective.Video signals, derived from the television camera tube, are applied tothe one end of the delay line through a diode biassedsignal-level-dependent circuit adapted to differentiate against highlevel signals, and video signals are also applied to the other end ofthe delay line through a similar diode biassed circuit adapted todifferentiate against low level signals. Means are provided for takingoff output video signals from the end of the line which is terminated inthe characteristic impedance of the line. The bandwidth for videosignals corresponding to low luminance areas of a subject oftransmission is reduced compared to the bandwidth for video signalscorresponding to higher luminance areas.

This invention relates to television cameras.

As is well known normally available television picture reproducing tubeshave markedly nonlinear characteristics and it is usual practice,therefore, to provide contrast law correction in a television camera inorder to provide compensating correction for such nonlinearity. Thecontrast law correction increases the small signal gain in regions oflow signal level (blacks") with respect to that in regions of highsignal level (whites"). A ratio of gain of 8 I from black to white iscommon. Such contrast correction has, however, the defect that it causesan undesirable increase of noise in the black region and it is commonpractice therefore, to overcome or reduce this defect by the expedientof providing means controlled by the level of the video signals, forvarying the bandwidth of a channel interposed in the path of saidsignals so as to reduce the bandwidth for video signals corresponding tolow luminance areas of a subject of transmission as compared to that forvideo signals corresponding to higher luminance signals. Although theadoption of this expedient involves reduction of bandwidth, this, andthe loss of resolution which accompanies it, are not seriouslyobjectionable, because owing to the reduced acuity of human vision forlow luminance areas of a picture the theoretical loss of resolutionwhich accompanies the adoption of the expedient in question is muchoutweighed from the practical point of view by the reduction of noise"in low luminance areas.

The present invention seeks to provide improved and simple means forvarying the bandwidth of a video signal channel under the control of thevideo signal level so as to reduce the bandwidth for video signalscorresponding to low luminance areas of a subject of transmission ascompared to the bandwidth video signals corresponding to higherluminance areas.

According to this invention video signals derived from a televisioncamera tube are applied to one end of a delay line which is reflectingat one end and terminated to be nonreflecting at the other, throughsignal-level-dependent means adapted to differentiate against high levelsignals and video signals are also applied to the nonreflecting end ofthe line through signal-level-dependent means adapted to differentiateagainst low level signals, output signals being taken from the line.

The invention has the important practical advantage (in addition to thatof simplicity) that the required differentiation between signals ofdifferent levels is obtained without introducing to any appreciableextent, undesirable frequencydependent phase shifts, so that theexpensive additional phasecorrecting circuitry usually necessary inknown comparable arrangements are not required.

One embodiment of the invention comprises a delay line which isreflective at one end and terminated by an impedance equal to itscharacteristic impedance at the other; a first transistor by means ofwhich video signals are applied to the reflective end of the line and towhich video signals are applied through a circuit including a resistanceshunted by a diode sensed and biassed to cut off low signal levels; asecond transistor by means of which video signals are applied to thecharacteristic impedance terminated end of the line and to which videosignals are applied through a circuit including a resistance in serieswith a diode sensed and biassed to cutoff high signal levels; and meansfor taking off output video signals from the characteristicimpedance-terminated end of the line.

The invention is illustrated in and further explained in connection withthe accompanying drawings in which:

FIG. 1 is a diagram of a preferred embodiment;

FIG. 2 is an explanatory vector diagram; and

FIG. 3 is an explanatory graphical figure.

Referring to FIG. 1 a delay line, comprising series inductors and shuntcapacitors, is terminated at one end by a resistance 2 equal to itscharacteristic impedance. There are two transistors 3 and 4 which are asnearly as possible alike and of the same gain. Video signals are appliedat IN through a resistance 5 shunted by a diode 6, sensed as shown, tothe base of the transistor 3 the collector of which supplied videosignals to the open end of the line 1. Video signals are also appliedfrom IN through a second diode 7, sensed as shown, in series with aresistance 8 to the base of the transistor 4 the collector of whichsupplied video signals to the end of the line terminated by theresistance 2. A tap 9 on a potentiometer comprising resistances l0 and11 connected as shown is connected to one end of the series circuitportion comprising the elements 7 and 8 the other end of this circuitportion being connected to the input terminal IN. Output is taken off atthe output terminal OUT.

To understand the operation of this embodiment ignore for the moment thetwo diodes 6 and 7 and the two associated resistances 5 and 8 supporttwo fractions a and b of a video signal voltage V to be appliedsimultaneously to the bases of the transistors 3 and 4, so that theinput voltage to 3 is a V and that to 4 is b V Then the output voltage(V at OUT across resistances 2 (of value R will be made up of:

1. a voltage a C'V -R due to a current i from transistor 3 and flowingalong the line to R and delayed by a time T clue to the line G is thegain of transistor 3 and equals that of transistor 4.

2. a voltage rb GV -R due to half of the current i from transistor 4 andflowing, undelayed, in resistance 2.

3. a voltage bb GV R, other half of the current i 2 which flows to thereflective end of the delay line, is reflected back therefrom and istherefore delayed by a time 2T.

The resultant combination of these components to produce V is shown byFIG. 2. The resultant current in resistance 2 (R is GV, (a+b cos 0)where 0 wT and w 211], fbeing the frequency. As will be seen, becausethe two vectors 'rb GV make equal and opposite angles with the vector aCV the output voltage is in phase with the component aGV at allfrequencies and all values of a and b. The output voltage magnitude V,is given by V,,= GV R (a cos 0) GV -R, (a b cos wt) Condition A. If a bk the output voltage is G V R at DC. When wT= /2 1r i.e. cos WT= 0 theoutput voltage is 5% G V R When wT=1r i.e. cos WT=-l the output is zero.FIG. 3 graphically illustrates this condition. In practice the portionof FIG. 3 to the right of 0 11' and f 0 is ignored because the overallbandwidth employed in the television system of which the camera formspart excludes the said portion, cutting off at (about) where 0 11' and f0.

Condition B. If a 1 and b 0 the output voltage is G V R at allfrequencies.

Accordingly, the required level-dependent bandwidth control can beachieved by making the circuit change from condition A to condition B atsome chosen signal level. This is achieved in the embodiment of FIG. 1by means of the circuits including the diodes 6 and 7. At low signallevels diode 6 is biassed to cut off, diode 7 conducts and the circuitoperates in condition A. When the signal level increases to a chosenvalue 6 conducts and 7 cuts off, and the circuit operates in conditionB. The point at which the circuit changes from one operating conditionto the other can be adjusted by adjusting the DC level of the input atIN.

I claim:

1. A circuit for receiving video signals from a television camera tubecomprising a delay line which is reflective at one end and nonreflectiveat its other end, a first signal level dependent means for cutting offsignals below a predetermined level, a second signal level dependentmeans for cutting off signals above a predetermined level, means forapplying signals through said first signal level dependent means to saidreflective end of the delay line, means for applying signals throughsaid second signal level dependent means to said nonreflective end ofthe delay line, and means for taking off video signals from the delayline.

2. A circuit as claimed in claim ll wherein said first signal leveldependent means comprise a resistance shunted by a diode sensed andbiassed to cut off low level signals, and said second signal leveldependent means comprise a second resistance in series with a seconddiode sensed and biassed to cut off high signal levels.

3. A circuit as claimed in claim 2 further comprising a first transistorhaving its input means coupled to the first said resistance shunted bythe first said diode, the first transistor having its output meanscoupled to the reflective end of said delay line; and a secondtransistor having its input means coupled to a circuit comprising theseries connected second resistance and second diode, the secondtransistor having its output means coupled to the nonreflective end ofsaid delay line.

l. A circuit as claimed in claim 1 wherein said other end of the delayline is terminated by an impedance equal to the characteristicimpedance.

5. A circuit as claimed in claim 2 wherein said other end of the delayline is terminated by an impedance equal to the characteristicimpedance.

6. A circuit as claimed in claim 3 wherein said other end of the delayline is terminated by an impedance equal to the characteristicimpedance.

7. A circuit as claimed in claim 4 wherein said means for taking delayvideo signals from the delay line is connected to said other end of thedelay line.

8. A circuit as claimed in claim 5 wherein said means for taking offvideo signals from the delay line is connected to said other end of thedelay line.

9. A circuit as claimed in claim 6 wherein said means for taking offvideo signals from the delay line is connected to said other end of thedelay line.

1. A circuit for receiving video signals from a television camera tubecomprising a delay line which is reflective at one end and nonreflectiveat its other end, a first signal level dependent means for cutting offsignals below a predetermined level, a second signal level dependentmeans for cutting off signals above a predetermined level, means forapplying signals through said first signal level dependent means to saidreflective end of the delay line, means for applying signals throughsaid second signal level dependent means to said nonreflective end ofthe delay line, and means for taking off video signals from the delayline.
 2. A circuit as claimed in claim 1 wherein said first signal leveldependent means comprise a resistance shunted by a diode sensed andbiassed to cut off low level signals, and said second signal leveldependent means comprise a second resistance in series with a seconddiode sensed and biassed to cut off high signal levels.
 3. A circuit asclaimed in claim 2 further comprising a first transistor having itsinput means coupled to the first said resistance shunted by the firstsaid diode, the first transistor having its output means coupled to thereflective end of said delay line; and a second transistor having itsinput means coupled to a circuit comprising the series connected secondresistance and second diode, the second transistor having its outputmeans coupled to the nonreflective end of said delay line.
 4. A circuitas claimed in claim 1 wherein said other end of the delay line isterminated by an impedance equal to the characteristic impedance.
 5. Acircuit as claimed in claim 2 wherein said other end of the delay lineis terminated by an impedance equal to the characteristic impedance. 6.A circuit as claimed in claim 3 wherein said other end of the delay lineis terminated by an impedance equal to the characteristic impedance. 7.A circuit as claimed in claim 4 wherein said means for taking delayvideo signals from the delay line is connected to said other end of thedelay line.
 8. A circuit as claimed in claim 5 wherein said means fortaking off video signals from the delay line is connected to said otherend of the delaY line.
 9. A circuit as claimed in claim 6 wherein saidmeans for taking off video signals from the delay line is connected tosaid other end of the delay line.